I haven't noticed any pattern to what causes it; just that sometimes, Firefox stops responding to key presses of any kind. I know some websites intercept keys that would otherwise control the browser (which is annoying), but when this happens it affects all sites. I've tried pressing all the modifier keys to see if one is being registered as held down; it doesn't help. Quitting Firefox and starting it again seems to fix it when it happens. I've been using Firefox for a while, and only encountered the problem once I installed it on my new Windows 8 PC.